Now that's a nice question.
If you hold an apple up in certain ways, it feels like you're doing work but in other ways it doesn't. If you lie on the ground with an apple on your belly (or hand) it doesn't feel like you're doing work. If you hold the apple out in your hand, it does feel like you're doing work. There must be some difference.
of doing work is a way of registering what's going on in your muscles. If they're using up chemical fuel to make one muscle protein (actin) move with respect to another (myosin) then you sense that you're doing work. Yet if the apple is just staying at a fixed height, no work is being done on it, in the physics sense. Why do you need to use energy in your muscles just to keep the apple up?
If you're using your muscles to hold up the apple, it would slip a little because the actin and the myosin don't stick perfectly. You have to use up some energy to make up for that slippage.
Here's an analogy. Say you have a car parked on a slope, with very high friction between the tires and the road. The car doesn't need to do any work to stay up. You can leave the engine off. Now say that the tires slipped a little on the road. The car will fall unless you turn on the engine and get the wheels turning, expending fuel. Keeping the car in place will feel a lot like accelerating it, since the motor will vibrate, fuel will be used up, exhaust products made, etc.
Now back to the magnet. There aren't any little molecules slipping when the magnet sticks well. It's like the car with high-friction tires. So no energy has to be expended.
(published on 12/06/11)